Field emission cathodes and arrays of field emission cathodes are known electron beam sources for electron beam devices in applications as diverse as e.g. electron microscopy, electron pattern generators or flat panel displays.
Field emission cathodes emit electrons into free space by applying a high electric field to the surface of the emitter tip of the field emission cathode. Without electric field there is usually a potential barrier of theoretically infinite thickness at the interface of the emitter tip and free space or vacuum.; The height of the potential barrier depends on the surface material of the emitter tip. When an external electric field is applied to the emitter tip that attracts electrons, the potential barrier thickness reduces. When the electric field at the surface of the emitter tip is larger than ca. 108 V/m, the potential barrier thickness reduces to a level where electrons in the emitter tip succeed in tunneling through the potential barrier into free space. This phenomenon is called field emission, in contrast to electron emission caused by e.g. thermal excitation, photo-effect etc.
Usually the high electric field is generated by applying a voltage between the emitter tip and an extracting electrode facing the emitter tip. In order to achieve sufficient field strength at the emitter tip, the electron emitting surface of the emitter is in the shape of a sharp tip (tip radius typically 1 nm to 100 nm). The emitter tip is usually made of metal or semiconductor material.
Among the many advantages of field emission cathodes compared to more traditional electron beam sources, like e.g. tungsten hairpin filaments, are their small emission source size, which is important for electron beams-used for precision focussing applications, their superior brightness, a smaller energy spread of the electrons within the electron beam and a longer lifetime. However, field emission cathodes also have drawbacks because of their need for high vacuum and because of a poor electron emission current stability.
The electron emission current instability is understood to be caused by the extreme sensitivity of the electron emission current on chemical or physical changes of the surface of the emitter tip. With the emitter tip having an apex radius of typically only a few nanometers, a deposition of a few atom layers or tiniest deformations of the apex during operation can cause significant electron emission current changes during operation. Many applications like e.g. electron microscopy, e-beam pattern generators, and other precision devices, require a high electron beam current stability.
To achieve a better electron emission current, some effort has been made to actively regulate the electron emission current by adjusting the voltage between emitter tip and extracting electrode according to the changes of the electron emission current. However this concept has the drawback that for electron beam precision devices like electron microscopes, the voltage changes between extracting electrode and emitter tip interfere with the electric field of the electron beam optics. Such interference can deteriorate the focussing capabilities of precision electron beam devices.
For some time large arrays of field emission cathodes have been integrated onto semiconductor substrates using semiconductor microprocessing techniques. Semiconductor microprocessing techniques allow large arrays of micron-size field emission cathodes to be fabricated onto minimal surface area. In addition, extraction electrodes and/or electronic control circuits for each field emission cathode can be integrated onto the semiconductor substrates in a cost-effective way. Arrays of field emission cathodes are seen to have large commercial potential for many applications, e.g. for flat panel displays as well as for electron microscopy or e-beam pattern generators where parallel operating electron beams can dramatically improve the processing throughput.
The fabrication of field emission cathodes on semiconductor material has several advantages. One reason is that the fabrication of emitter tips from a semiconductor substrate, especially from silicon substrates, is straightforward. Furthermore, semiconductor emitter tips can be doped in order to adjust their electronic properties to a given application. In particular it has been found that the choice of the polarity of the majority carrier of the respective semiconductor material has a profound impact on the emission behavior of emitter tips: n-type semiconductor emitters connected to some voltage source like metallic emitters emit electrons according to the Fowler-Nordheim formula; in contrast p-type emitters connected to some voltage source deviate from the Fowler-Nordheim formula significantly.
The different electron emission current behavior of p-type emitters is thought to be caused by the absence of electron abundance in p-type emitters. Therefore, the emission current can be limited by the number of free electrons in the p-type material, and not by the potential barrier at the surface of the emitter tip. This is contrary to the model of Fowler-Nordheim, where the electron emission current is limited by the potential barrier at the emitter surface.
A detailed study of the different behaviors of p-type emitters and n-type emitters has been performed, e.g. in “Control of emission currents from silicon field emitter arrays using a built-in MOSFET” by Seigo Kanemaru et.al., Applied Surface Science 111 (1997),p.218-223 or “The Semiconductor Field-Emission Photocathode” by Dieter K. Schroder et.al., IEEE Trans. Electr. Dev., Vol ED-21, No 12, Dec. 1974.
In “The Semiconductor Field-Emission Photocathode” by Dieter K. Schroder et.al., the electron emission current limiting effect has been used to design a p-type field emission cathode where the emission rate is controlled by external light that generates electron-hole pairs in the p-type emitter region via photo-effect. The generated electrons diffuse until they recombine or arrive at the emitter surface where they can be emitted with an external electric field. The strength of the external field is so high that, in this model, the emission current is limited by the number of free electrons generated by the external light intensity and not by the tunneling probability through the potential barrier.
An important advantage of generating an electron beam current through light excitation is that the electron beam current can be controlled by the external light intensity without changing the voltage between extracting electrode and emitter tip. This avoids the mentioned problem of interfering with the electric fields of the electron beam optics used for high precision electron beam devices.
The p-type field emission cathode with light excitation however has severe limitations. For one thing, it is costly to install a light source near a field emission cathode with a beam that points to the small emitter tip region. Even when light is coming from behind of the substrate as shown in the above-mentioned paper by D. Schroder, it is difficult to control the stability of the light power to the extent needed for a well-controlled electron beam current. Finally, for a large array of field emission cathodes integrated onto a substrate there seems no easy way to control the emission current individually by the use of external light sources.